The invention relates to a new hydrodehalogenation catalyst and to its use for the hydrodechlorination of chlorinated hydrocarbons. The catalyst is suitable for hydrodehalogenation under mild reaction conditions, such as a temperature of 30.degree. to 230.degree. C. and pressures from atmospheric to 8 bar.
Chlorinated hydrocarbons are a by-product of numerous processes of the chemical industry. Because of the high toxicity of these compounds, it is necessary to eliminate them from halogenated organic waste. For this purpose, noncatalytic methods, such as the treatment with metallic sodium (DEGUSSA Method), the high temperature combustion, UV irradiation and adsorption methods, as well as catalytic methods have been used. The predominant catalytic methods are the heterogeneous catalytic methods, the hydrogenating dechlorination and the catalytic combustion.
The danger of forming dioxins and the combustion of valuable hydrocarbons are disadvantages of the catalytic combustion methods. For this reason, there has been more emphasis on methods for the hydrogenating disposal of chlorinated hydrocarbons.
One such method is the hydrogenating conversion of chlorobenzenes on nickel-containing catalysts, as described in the patents RO 61 442 and DD 294 965. However, the catalysts used here have the disadvantage that they cannot be reactivated.
The hydrogenating dechlorination of chlorinated hydrocarbons on rhodium support catalysts is described in DE 2 164 074. At a loading of approximately 0.1 v/vh at 180.degree. C., atmospheric pressure and a molar ratio of dichloropropane to hydrogen of 1:3.5, it was possible to achieve complete conversion over a period of 400 hours. The high costs of rhodium are, however, a disadvantage of this catalyst system.
The use of a catalyst consisting of platinum, palladium, rhodium, ruthenium and/or mixtures of these on an iron and/or nickel basic alloy with a chromium content of at least 15% and coated with titanium dioxide is protected in the German patent 4 200 790. The lifetime of these catalysts was not given in this patent. Moreover, conversions are incomplete even at temperatures of 250.degree. C.
The CS 267 472 patent discloses the catalytic hydrogenation of hexachlorobenzene to cyclohexane in the presence of palladium supported catalysts containing 0.1 to 5% by weight of palladium, carbon, silica or alumina being used as support. However, these catalysts preferably work only at temperatures from 220.degree. to 280.degree. C.
The U.S. Pat. No. 3,855,347 discloses a catalyst, which was produced by impregnating silica/alumina with a H.sub.2 O-HCl-PdCl.sub.2 solution. This catalyst dehydrochlorinated 1,2-dichloropropane. However, the reaction was incomplete even at 320.degree. C.
Moreover, the DE-AS 1 036 830 describes Pd-Al.sub.2 O.sub.3 -SiO.sub.2 catalysts, which were prepared by suspending support gels in palladium nitrate solutions, for the hydrogenation of alkylated or arylated anthraquinones. However, this method of manufacture does not permit the distribution of the palladium to be adjusted selectively over the cross section of the catalyst castings.
The hydrogenation dechlorination of chloromethanes in the presence of a noble metal-containing supported catalyst with an alumina support, which preferably has a surface area of 5-90 m.sup.2 /g, up to 0.1% of silica and up to 2% of other metal oxides such as TiO.sub.2, Fe.sub.2 O.sub.3 and Na.sub.2 O, is described in the EP 570 050. With this method also, only partial conversion is achieved, as is evident from the examples given.
As has already been shown partly, all the catalysts mentioned have a series of disadvantages. For example, the use of some catalysts is restricted to only a certain group of chlorinated hydrocarbons, and/or the working temperature of the catalyst lies above 230.degree. C. In particular, the lifetimes of the known catalysts are unsatisfactory, the longest one described being 400 hours.